The Rise of Oxygen and the early animals.

Iron formation from the Pongola Supergroup, South Africa. Credit: Nic Beukes/Univ. of Johannesburg.

Earth is the only planet in our Solar System with high concentrations of gaseous diatomic oxygen. Simultaneously, this unique feature of Earth’s atmosphere has allowed the presence of an ozone layer that absorbed UV radiations. But the oxygen content of Earth’s atmosphere has varied greatly through time. For about the first 2 billion years of Earth’s history, the atmospheric oxygen concentration was exceptionally low.

It’s widely assumed that about 2.3 billion years ago, the level of oxygen increased dramatically in a process called the Great Oxidation Event (GOE). This rise in oxygen level occurred during an episode of major glaciation known as the Huronian glaciation. The progressive oxygenation of the atmosphere and oceans was sustained by an event of high organic carbon burial, called the Lomagundi Event, which lasted well over 100 million years, and represents the largest positive carbon-isotope excursion in Earth history (Canfield, 2013). This early oxygen primary production  was exclusively conducted by prokaryotes, specifically by cyanobacteria.

Precambrian stromatolites in the Siyeh Formation, Glacier National Park. From Wikimedia Commons.

However, new geochemical  evidence suggested that there were appreciable levels of atmospheric oxygen about 3 billion years ago, more than 600 million years before the Great Oxidation Event, indicating a greater antiquity for oxygen producing photosynthesis and aerobic life.

After the GOE, oxygen levels rose again and then fell in the atmosphere and remained at extremely low levels for more than a billion years. This was probably due to a particular combination of  biogeochemical feedbacks that spawned an oxygen-lean deep ocean (Lyons, 2014). The general oxygenation of the oceans began around 750-550 million years ago. This recovery  of oxygen levels led to a significant increase in trace metals in the ocean and possibly triggered the ‘Cambrian explosion of life’ (Large, 2014).

Halichondria panicea, a temperate marine demosponge (Photo: Daniel Mills)

But early animals, in general, may have had relatively low oxygen requirements. According to new findings, a sea sponge – the living animal that most resembles the earliest animals on Earth – can live and grow even at atmospheric oxygen levels that are 0.5 percent of today’s levels, which challenges the notion that low oxygen levels were the limiting factor for animal evolution. The study also suggest that the evolution of sophisticated gene regulatory networks, may have controlled the timing of animal origins more so than environmental parameters  (Mills, 2014)

References:

Donald E. Canfield, Lauriss Ngombi-Pemba, Emma U. Hammarlund, Stefan Bengtson, Marc Chaussidon, François Gauthier-Lafaye, Alain Meunier, Armelle Riboulleau, Claire Rollion-Bard, Olivier Rouxel, Dan Asael, Anne-Catherine Pierson-Wickmann, and Abderrazak El Albani,  Oxygen dynamics in the aftermath of the Great Oxidation of Earth’s atmosphere PNAS 2013 110 (42) 16736-16741; published ahead of print September 30, 2013, doi:10.1073/pnas.1315570110.

Daniel B. Mills, Lewis M. Ward, CarriAyne Jones, Brittany Sweeten, Michael Forth, Alexander H. Treusch, and Donald E. Canfield, Oxygen requirements of the earliest animals, PNAS 2014 ; published ahead of print February 18, 2014, doi:10.1073/pnas.1400547111

Timothy W. Lyons, Christopher T. Reinhard, Noah J. Planavsky. The rise of oxygen in Earth’s early ocean and atmosphere. Nature, 2014; 506 (7488): 307 DOI: 10.1038/nature13068

Diatoms and Climate Change.

Diatoms living between crystals of annual sea ice in McMurdo Sound, Antarctica. From Wikimedia Commons.

Diatoms are unicellular algae with golden-brown photosynthetic pigments with a fossil record that extends back to Early Jurassic. The most distinctive feature of diatoms is their siliceous skeleton known as frustule that comprise two valves. The formation of this opaline frustule is linked  in modern oceans with the biogeochemical cycles of silicon and carbon.

Because their abundance and sensitivity to different parameters,  diatoms play a key role in Paleoceanography , particularly for evidence of climatic cooling and changing sedimentation rates in the Arctic and Antarctic oceans and to estimate sea surface temperature. Also, diatom diversity can be used as a proxy for the influence of diatoms on marine export productivity and the carbon cycle.

Diatoms are thought to have diversified over the Cenozoic. Early Cenozoic oceans were relatively warm, but in the early to mid Eocene, ocean surface temperatures began to cool, and polar regions and tropical regions began to be more strongly differentiated. It was suggested that Late Eocene diatom proliferation likely occurred in response to subsidence of Southern Ocean land bridges and the concurrent development of circum-Antarctic upwelling.

Actinocyclus ingens Rattray and Thalassiosira convexa (SEM, Neogene diatoms from the Southern Ocean, ODP)

Peak species diversity in marine planktonic diatoms occurred at the Eocene–Oligocene boundary followed by a pronounced decline, from which they have not recovered (Rabosky 2009).  During the early late Miocene, when temperatures and pCO2 were only moderately higher than today, diatoms lost about 20% of its diversity. Warmer oceans are linked with lower diatom diversity, suggesting that future warmer oceans due to anthropogenic warming may result in lower diatom diversity (Lazarus, 2014).

During the last 15 million years, diatom diversity is correlated with global carbon isotope record and with the past atmospheric pCO2, suggesting that diatoms have played a very important role in the evolution of mid-Miocene to Recent climate for their prominent role in the carbon pump.

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Lazarus D, Barron J, Renaudie J, Diver P, Türke A (2014) Cenozoic Planktonic Marine Diatom Diversity and Correlation to Climate Change. PLoS ONE 9(1):e84857. doi:10.1371/journal.pone.0084857

Egan KE, Rickaby REM, Hendry KR, Halliday AN (2013) Opening the gateways for diatoms primes Earth for Antarctic glaciation. Earth and Planetary Science Letters 375: 34–43. doi: 10.1016/j.epsl.2013.04.030